In recent years, color liquid crystal display devices have been widely used in various fields as monitors of portable notebook personal computers, persons computers, and so forth, or as display portions of liquid crystal televisions, video-combined liquid crystal televisions, portable telephones, portable information terminals or personal digital assistants, and so forth. Further, following increase in information processing amount, diversification of needs, multimedia adaptation, and so forth, increase in screen size and fineness in display of the liquid crystal display devices has been actively developed.
The liquid crystal display device basically comprises a backlight portion and a liquid crystal display element portion. As the backlight portion, there is an under-light type wherein a light source is disposed directly under the liquid crystal display element portion, or an edge-light type wherein a primary light source is disposed facing a side end surface of a light guide. In terms of making the liquid crystal display device compact, the edge-light type has often been used.
In recent years, in the liquid crystal display device or the like, use has been made of, in terms of reducing power consumption, one, as a backlight portion of the edge-light type, that reduces a spread angle of light emitted from a display screen as such as possible to emit light concentratedly in a required angle range for effectively utilizing the quantity of light emitted from a primary light source.
In the display device wherein the viewing direction range is limited as described above, as a light source device that emits light concentratedly in a relatively narrow range for increasing the utilization efficiency of the quantity of light of a primary light source to thereby reduce the power consumption, the present applicant has proposed in JP (A)-2001-143515 to use a prism sheet having prism-formed surfaces on both sides thereof so as to dispose it adjacent to a light outgoing surface of a light guide. In this double-sided prism sheet, a light receiving surface being one surface and a light emitting surface being the other surface are respectively formed with a plurality of mutually parallel elongated prisms, wherein the directions of the elongated prisms are matched with each other on the light receiving surface and the light emitting surface and further the elongated prisms are disposed at corresponding positions on both surfaces. With this configuration, light having a peak of outgoing light in a direction inclined relative to the light outgoing surface of the light guide and exiting from the light outgoing surface so as to be distributed in a proper angle range is incident on one-side prism faces of the elongated prisms at the light receiving surface of the prism sheet, then is internally reflected by the other-side prism faces, and is further subjected to a refraction action by the elongated prisms at the light emitting surface, so that the light is emitted concentratedly in a required relatively narrow direction.
However, such a light source device has a problem that although the concentrated emission in the narrow angle range is enabled, an outgoing light distribution is extremely narrow so that a large reduction in luminance is caused only by a small change in viewing direction, thereby being inferior in practicality in case of a light source device with a relatively large size. Further, according to such a light source device, in the prism sheet used as a light deflection element, the plurality of mutually parallel elongated prisms are required such that the directions of the elongated prisms are matched with each other on the light receiving surface and the light emitting surface, and further, the elongated prisms are disposed at the corresponding positions on both surfaces. This formation is complicated.
On the other hand, for the purpose of enhancing a light condensing property and light directivity when deflecting light emitted from a light guide by the use of a prism sheet, JP(A)-9-507584, JP(A)-9-105804, JP(A)-11-38209, and JP(A)-2000-35763 have proposed that a prism face, located on the side farther from a light source, of each of elongated prisms forming the prism sheet be formed into a convex curved surface shape. However, since the convex curved surface shaped prism faces described in those publications each have a radius of curvature that is relatively larger or relatively smaller, a distribution of the outgoing light from the light guide cannot be sufficiently narrowed or is extremely narrowed or, according to circumstances, is conversely broadened. Further, in any of the light guides described in those publications, a light emitting function structure thereof cannot achieve a high directivity so that light of which an outgoing light distribution is relatively broad is emitted, and therefore, it is not possible to sufficiently improve the luminance even by condensing the light using the prism sheet.
Further, JP(A)-63-298902 discloses a prism sheet having a light receiving surface formed with elongated prisms each having a pentagonal shape in section. A face on the side of a prism vertex portion and a face on the side near a light emitting surface, which constitute each of two prism faces of each elongated prism, form a large angle therebetween, and therefore, this prism sheet cannot sufficiently narrow a distribution of outgoing light from a light guide so that its light condensing property is low and thus a luminance improving effect thereby is small.